Decades of research have shown that the peptide oxytocin (OT) can act as a potent neuromodulator in a variety of species to influence complex social behaviors, including social bonding, affiliation, and social reward. Administering intranasal OT to humans affects a suite of social behaviors, such as trust, eye contact, emotion recognition, and pair-bonding-related behaviors. Due to the ability of OT to modulate social function in animals as well as humans, the OT system has been highly implicated in the biology and treatment of several psychiatric conditions that are characterized by deficits in sociality, including autism spectrum disorder, schizophrenia, and social anxiety disorder. Because of this high translational potential for OT to benefit human health, it is crucial that research efforts focus on the fundamental neuroanatomy and physiology of the oxytocin system in the brains of both animals and humans. Thanks to the suite of transgenic tools available, research in mice has contributed considerably to our understanding of the function of OT in the regulation of social behavior. But non-mouse models are increasingly being used, including monogamous rodents as well as nonhuman primates. To complement the elegant behavioral pharmacology being done in these species, rigorous neuroanatomical work is required to characterize the underlying neural circuits. Currently, the most reliable and widely available technique for the visualization of OXTR in brain tissue sections is receptor autoradiography, but this method has some limitations. It only resolves receptors at the gross anatomical level; it is not possible to analyze receptor expression on the cellular scale. The most common technique to visualize receptors on the cellular level is with a method called immunohistochemistry. But because there are no reliable, commercially-available antibodies for OXTR, the field of OXTR research has been left without a widely available and tractable technique to investigate these receptors on the cellular level. Thus, the first aim of our proposal is to advance the field of OT research by developing a novel method for the cellular staining of OXTR in brain tissue. This technique will use a novel biotinylated OXTR ligand provided by our chemist collaborator and will be optimized from prairie voles, titi monkeys, and humans. The second limitation of receptor autoradiography is that it uses an indirect visualization method (radiosensitive film) that doesn't label the tissue directly. Thus, it is impossible to perform co-localization studies with targets of other neurotransmitter systems known to interact with OT/OXTR to modulate social function. Aim 2 seeks to resolve this issue by applying the novel OXTR staining method to the localization of OXTR on dopaminergic neurons in prairie voles. Our decision to start with the dopaminergic system in prairie voles is based on extensive experimental evidence for the interaction between these two systems in the regulation of pair bond formation in this monogamous rodent. By developing a cellular stain for OXTR that is tractable across laboratories and species, a massive opportunity will be created for future studies of OXTR-expressing neurons.